I have investigated the subject about a decade ago. All my findings are from Ram Krishna Math and its associates.
The Album was published by Ramkrishna Mission on the Centenary year of Swami Vivekananda.
Ramakrishna Mission did not mention in the inlay card that the voice is someone else's.
When asked, the publishers explained the matter as some legal complications. They did not give a reason for this at that time.
The voice is of N. Viswanathan, the famous English professor of St. Xaviers College Calcutta.
I cannot remember now which monk gave this information but certainly he was associated with the work.
Later on one of the students of Prof. N Viswanathan confirmed that it was him. About ten years back I shared a taxi with two other gentlemen. They were discussing the said subject. When asked, one of them told me that he was a student of Prof. N Viswanathan. He confirmed that Mr. Viswanathan read out the speech.
credits Subir Ghosh as the reader which seems to be incorrect.
There is no recorded voice of Swamiji. This has been confirmed by the monks of the Ramakrishna Mission who were involved in the centenary publications. They searched and probed for the same during the speech's centenary celebration. BBC once confirmed RK Mission that they don't have any recording of this kind.